But when you can't have an absolutely huge box (at least 20ft3), I'd prefer bass-reflex boxes.
This is exactly what is pushing me from the beginning to wondering about horns , TQWT, BIB, or other kind of design. The only place I dispose to put speakers in are two boxes, each of 15cu ft (67" x 18" x 25" w x h x d, they are indeed fixed on the rear wall at the 10" from floor). I'd like to use all available space to get a good bass out of 2225.
If I understand correctly your recommendation is then to use a classical 4Pi (slightly modified dimensions to fit in) and to use the remaining ~10 cuft per side for the additional sub-woofers? Any recommendation on such a subs? (I suppose we are talking completely different budget here...)